Mr Raj keeps red, blue and green building blocks. For each green block, there are 2 red blocks. 18 of all the blocks are blue. What fraction of Mr Raj's blocks are green?
Need a hint?
Find the fraction left after taking away the blue blocks. This fraction stands for 3 equal red-and-green parts, so divide it by 3.
Worked answer
- Treat the green blocks as 1 equal part.
- The red blocks make 2 equal parts, so the red and green blocks together make 3 equal parts.
- Blue blocks are 18 of all blocks, so the red and green blocks make 78 of all blocks.
- Each equal part is 78 ÷ 3 = 724, so the fraction of green blocks is 724.
Answer: 7/24 fraction
The mistake this question tests
Habit
Fraction of a remainder
Some pupils may use 18 for green because they confuse the blue fraction with one equal part. Others may use 78 without dividing it among the 3 red-and-green parts.
How to fix it at home →Wrong answers children often give:
- 1/8 Stopped at one part in ratio
- 7/8 Fraction of a remainder
- 7/3 Calculation slip
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See the answer →Practise PSLE fractions until the mistakes stop.
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